The present invention relates to an epoxy resin composition useful in preparation of an electrical laminate. More particularly, the present invention relates to an epoxy resin composition for use in preparation of an electrical laminate, which exhibits low viscosity when dissolved in an organic solvent, high glass transition temperature and good resistance to bromine release when cured.
Epoxy resins have been used as materials for electrical/electronics equipment, such as materials for electrical laminates because of their superiority in heat resistance, chemical resistance, insulation properties, dimensional stability, adhesiveness and the like.
Most electrical laminates are made from a varnish comprising a brominated epoxy resin prepared from the reaction of a certain amount of diglycidyl ether of bisphenol A (DGEBA) and a certain amount of tetrabromobisphenol A (TBBA), a curing agent and an accelerator dissolved in an organic solvent. In general, to provide sufficient flame retardancy, the brominated epoxy resins should have a bromine content of 20 to 23 weight percent. Thus, the conventional brominated epoxy resins are usually prepared by reacting 35 to 38 weight percent of tetrabromobisphenol A (TBBA) and 62 to 65 weight percent of a diglycidyl ether of bisphenol A (DGEBA).
To increase heat resistance of the resultant electrical laminates, it is preferable to use a brominated epoxy resin having low molecular weight. However, in the above TBBA/DGEBA system, it is difficult to control the molecular weight of the resultant brominated epoxy resin since the bromine content should be kept in the above ranges.
Accordingly, to improve heat resistance, approximately 15 parts by weight of a multifunctional epoxy resin such as a cresol epoxy novolac resin, are added to the TBBA/DGEBA system to prepare electrical laminates having a glass transition temperature of 135.degree. C. However, the addition of the multifunctional epoxy resin disrupts the uniformity of reaction, leading to poor surface smoothness of the resulting prepreg. Attempts have been made to react the multifunctional epoxy resin with other components. However, this technique is disadvantageous since the resultant brominated epoxy resins have higher viscosity. Some multifunctional epoxy resins, having narrow molecular weight distribution, may not cause increases in viscosity. However, such resins are expensive and therefore not suitable for routine industrial use.
For ease of handling of an epoxy resin during preparation of prepreg, a solution containing the epoxy resin should preferably have lowest possible viscosity, but use the least possible amount of an organic solvent. However, as mentioned above, it is difficult to sufficiently decrease the viscosity of the conventional epoxy resin solutions or difficult to decrease the amount of the organic solvent employed.
As a method of decreasing molecular weight of the resultant brominated epoxy resins, without decreasing the bromine content, it is known to add a diglycidyl ether of TBBA to the conventional brominated epoxy resin prepared from the TBBA/DGEBA system. It is also known to supply a diglycidyl ether of TBBA to a reaction mixture of TBBA and DGEBA during the production of the conventional brominated epoxy resin so that a portion of the digylcidyl ether of TBBA reacts with the DGEBA and TBBA. Such techniques can reduce the molecular weight of the resultant brominated epoxy resin, leading to improved heat resistance and decrease in viscosity of the resin solution. However, these resins also have disadvantages since the bromine attached to the molecules of the epoxy resins is easily dissociated. Dissociated bromine remaining in a laminate results in poor adhesion to a conductive layer, which causes delamination or peeling from the layer.
In view of the deficiencies of the above-mentioned conventional epoxy resin compositions, it would be desirable to provide an epoxy resin composition useful in preparation of electrical laminates, which exhibits low viscosity when dissolved in an organic solvent, improved resistance to bromine release and improved heat resistance when cured.